Back to EveryPatent.com
United States Patent |
5,065,835
|
Richter
,   et al.
|
November 19, 1991
|
Motor vehicle with wheels driven via differential gearing or the like
Abstract
A motor vehicle with four driving wheels and driven differential gearing
for the drive, where (in the case of rotational-speed differences
occurring between the wheels) a control device intervenes to distribute
the drive power of an engine to the permanently driven wheels and/or the
other driven wheels. This intervention occurs for a particular holding
time which is progressively extended if a new intervention by the control
becomes necessary after a test disconnection of the respective
intervention. In addition, or alternatively, the control device can
respond more sensitively and thus more quickly to rotational-speed
differences of the wheels which may later occur during, or after, a test
disconnection.
Inventors:
|
Richter; Karl-Heinz (Kernen, DE);
Rollmann; Gerhard (Tamm, DE);
Struck; Helmut (Winnenden, DE);
Eilert; Gerd (Schwaikheim, DE)
|
Assignee:
|
Daimler-Benz AG (DE)
|
Appl. No.:
|
435995 |
Filed:
|
November 14, 1989 |
Foreign Application Priority Data
Current U.S. Class: |
180/197; 180/249 |
Intern'l Class: |
B60K 028/16 |
Field of Search: |
180/197,248,249
|
References Cited
U.S. Patent Documents
4681185 | Jul., 1987 | Hoernig et al. | 180/247.
|
4704541 | Nov., 1987 | Leiber | 180/197.
|
4781265 | Nov., 1988 | Weiler et al. | 180/197.
|
4809807 | Mar., 1989 | Leiber | 180/197.
|
Foreign Patent Documents |
3225459 | May., 1986 | DE.
| |
3527959 | Feb., 1987 | DE.
| |
3545544 | Oct., 1987 | DE.
| |
3721662 | May., 1988 | DE.
| |
Primary Examiner: Marmor; Charles A.
Assistant Examiner: Bompey; Mitchell
Attorney, Agent or Firm: Evenson, Wands, Edwards, Lenahan & McKeown
Claims
What is claimed:
1. A motor vehicle having wheels driven via differential gearing, sensors
for detecting wheel rotational-speed, and means for forcedly achieving
synchronism between the wheels, said means being automatically actuated
for predeterminable holding periods when rotational-speed differences
exceed predeterminable threshold values and configured for repeated
actuation when new occurrences of the predetermined threshold values of
the rotational-speed differences are detected, wherein said repeated
actuations automatically take place in one of two states comprising
successively extended holding periods and on occurrence of successively
lowered threshold values if time intervals between occurrences of said
threshold values and respectively preceding actuations of said synchronism
means decrease as a first state, and on occurrences of successively
lowered threshold values if time intervals between occurrences of said
threshold values and respectively preceding actuations of said synchronism
means decrease as a second state.
2. The motor vehicle according to claim 1, wherein the length of the time
intervals depends on successively occurring threshold value changes.
3. The motor vehicle according to claim 1, wherein the means determines
values of rotational accelerations and decelerations of the vehicle
wheels;
stores there determined values for a predeterminable time period; and
increases or decreases relative rotational-speed threshold values in
response to the stored values of the accelerations or decelerations at low
vehicle driving speeds.
4. The motor vehicle according to claim 2, wherein the means determines
values of rotational accelerations and decelerations of the vehicle
wheels;
stores these determined values for a predeterminable time period; and
increases or decreases relative rotational-speed threshold values in
response to the stored values of the accelerations or decelerations at low
vehicle driving speeds.
5. The motor vehicle according to claim 1, wherein at lower vehicle speeds,
a relative rotational-speed threshold value for a maximum permissible
rotational-speed difference between a front and a rear wheel is changed
inversely proportionally to a product of a respective steering angle of
the vehicle and driving speed of the vehicle.
6. The motor vehicle according to claim 2, wherein at lower vehicle speeds,
a relative rotational-speed threshold value for a maximum permissible
rotational-speed difference between a front and a rear wheel is changed
inversely proportionally to a product of a respective steering angle of
the vehicle and driving speed of the vehicle.
7. The motor vehicle according to claim 1, wherein when driving in a curve
with increasing driving speed and increasing steering angles, the means is
operatively configured to provide for an increased relative
rotational-speed threshold value for rotational speed differences between
steering wheels on opposite sides of the vehicle.
8. The motor vehicle according to claim 2, wherein when driving in a curve
with increasing driving speed and increasing steering angels, the means is
operatively configured to provide for an increased relative
rotational-speed threshold value for rotational speed differences between
steering wheels on opposite sides of the vehicle.
9. The motor vehicle according to claim 3, wherein when driving in a curve
with increasing driving speed and increasing steering angles, the means is
operatively configured to provide for an increased relative
rotational-speed threshold value for rotational speed differences between
steering wheels on opposite sides of the vehicle.
10. The motor vehicle according to claim 1, wherein after an actuation of
the means to reduce speed differences while the vehicle is turning, a
repeated action to reduce relative speed only occurs when vehicle steering
reaches or is close to a straight-ahead position.
11. The motor vehicle according to claim 2, wherein after an actuation of
the means to reduce speed differences while the vehicle is turning, a
repeated action to reduce relative speed only occurs when vehicle steering
reaches or is close to a straight-ahead position.
12. The motor vehicle according to claim 1, wherein during all-wheel drive,
a differential lock associated with a rear wheel axle of the vehicle is
switched on in response to rotational-speed differences of the front
wheels.
13. The motor vehicle according to claim 2, wherein during all-wheel drive,
a differential lock associated with a rear wheel axle of the vehicle is
switched on in response to rotational-speed differences of the front
wheels.
14. The motor vehicle according to claim 1, wherein during successive
starting attempts of the vehicle that follow within a predeterminable time
interval, the means is operatively configured to store information as to
actuated status of the respective locks, gears, brakes, clutch during a
preceding starting attempt and they remain effective for the next start
for the predeterminable time interval and to extend that time interval.
15. The motor vehicle according to claim 1, wherein said means controls
actuation of either a rear-axle differential lock or rear wheel brakes for
achieving synchronism between the speeds of the rear wheels in dependence
on steering angle and on the driving speed.
16. The motor vehicle according to claim 2, wherein said means controls
actuation of each a lock of a rear-axle differential locks or rear wheel
brakes for achieving synchronism between the speeds of the rear wheels in
dependence on steering angle and on the driving speed.
17. The motor vehicle according to claim 1, wherein the means samples
rotational wheel speeds during straight-ahead driving and one of
unactuated brakes and differential locks and checks the relative wheel
speeds within a predetermined speed or acceleration range, and
the means is configured so that no subsequent actuation by the means will
occur as long as the relative rotational-speed conditions remain constant.
18. The motor vehicle according to claim 2, wherein the means samples
rotational wheel speeds during straight-ahead driving and one of
unactuated brakes or differential locks and checks the relative wheel
speeds within a predetermined speed or acceleration range, and
the means is configured so that no subsequent actuation by the means will
occur as long as the relative rotational-speed conditions remain constant.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a motor vehicle with all wheels driven via
differential gearing or the like, wherein a control device samples the
rotational speed of the wheels via sensors and which--in order to achieve
synchronism between driving wheels, actuates locks associated with the
differential gearing for predeterminable holding times when
rotational-speed differences of the wheels exceed threshold values which
are permanently predetermined or predetermined in dependence on the
driving condition. The control can actuate the locks of the differential
gearing, or differential gears, or individual brakes associated with the
driving wheels, and/or a clutch arrangement connecting the connectable
all-wheel drive to the drive shafting of the vehicle to control the speed
of the driven wheels.
In addition to vehicles with permanent all-wheel drive, there are vehicles
in which the all-wheel drive is manually or automatically connected.
For automatic connection, viscous clutches are often used in which the
torque transmitted depends on the differences in rotational speed between
the clutch input and output. When the rotational-speed difference is
exceeded, the amount of transferable torque rises steeply. This makes it
possible for the vehicle to behave like a rear wheel-driven (or also
front-wheel-driven) vehicle when driving on a non-slip base. The all-wheel
drive only becomes effective under difficult driving conditions.
However, it is disadvantageous if the all-wheel drive becomes effective
with a permanently predetermined difference in rotational speed between
front and rear wheels and accordingly it is impossible to take into
consideration special driving conditions.
For this reason, there are automatically connectable all-wheel drives which
are operated by means of a control device which is responsive to
rotational wheel speeds sensors and to other parameters such as, for
example, steering angles or the like. This basically provides an
opportunity for controlling the all-wheel drive in an extremely flexible
manner. For example, the all-wheel drive can only be connected within a
limited speed range and when the vehicle is allowed to drive exclusively
in a predetermined driving mode (e.g. with rear-wheel drive or with
front-wheel drive at relatively high speeds). It is also possible to
connect the all-wheel drive at different driving speeds only after
different threshold values for the rotational-speed differences between
front and/or rear wheels are exceeded. As a result, the manageability of
the vehicle can be considerably improved in boundary situations.
In principle, the same applies to differential locks, for example those
which lock a central differential between front axle and rear axle in an
all-wheel drive, or the differential of a driving axle in vehicles having
only one driven axle or all wheel drive. Although locks can be basically
constructed as viscous clutches, arrangements in which the locks or their
locking degree are controlled by means of a control device in dependence
on the rotational wheel speeds (or the differences of the rotational wheel
speeds) are superior with respect to the possible variability of the
control system.
In the case of vehicles having only one driven axle, instead of providing a
differential lock, one can separately control wheel brakes in dependence
on the rotational wheel speeds (or on differences between the rotational
wheel speeds) by means of the control device in order to brake a driving
wheel which is spinning and thus rotating very quickly compared with the
remaining vehicle wheels. As a result, an increased torque is
automatically transmitted via the differential to the other driving wheel
so that a desired synchronism of the driving wheels can be produced.
In all systems in which the wheel brakes, and/or the differential locks,
and/or the clutch arrangement controlling the connectable all-wheel drive,
are actuated by means of a control device, it is advantageous if the
sensors, necessary for controlling the rotational wheel speeds, already
exist. Because vehicles equipped with such elaborate driving systems
usually also have a service brake with an anti-blocking system, the
necessary sensors are available without adding them. For others, control
devices with computer support can also be used for other tasks when there
is adequate computer capacity in such vehicles.
It is then an object of the invention to further develop a motor vehicle of
the above general type in such a manner that a particularly good
adaptation to respective driving conditions is rendered possible.
According to the invention, this object is achieved by having the control
device actuate the lock or locks, the brake or brakes, and/or the clutch
device after partial or complete disengagement of the latter (test
disconnection) in dependence on a variable time interval. The time
interval elapses after the partial or complete disengagement to a new
occurrence of the threshold value for progressively increased holding
times, and/or during a predeterminable time interval, even when a reduced
threshold value or reduced threshold values occur.
The invention is based on the general finding that especially difficult
roadway conditions (slippery) must be present if a new intervention
becomes necessary, immediately after an intervention of the control device
in the distribution of the drive power to the permanent, or connectable
drive wheels. In order to be able to perform a sensitive control, the
respective intervention can initially occur during a very brief holding
time which is then followed by a test disconnection. That is to say, the
previously actuated lock, brake and/or clutch arrangement are disengaged
and subsequently actuated for an increased holding time if the rotational
wheel speeds (or differences of these), which occur during the test
disconnection, exceed predeterminable threshold values. Due to the fact
that the holding time is increasingly extended up to a maximum value, one
avoids possible excitation of oscillations which are unwanted in the drive
shafting.
Additionally or alternatively, the response sensitivity of the control
device is increased after an intervention of the latter so that, under
certain circumstances, there is no complete test disconnection of the
brake, lock and/or clutch arrangement which was in each case previously
actuated. Instead the latter are already actuated when rotational speed
differences between the wheels of the vehicle are beginning or are still
relatively slight.
In the case of an automatically connected all-wheel drive, the drive wheels
are thus not even completely disconnected from the drive shafting during a
test disconnection. Instead, under certain circumstances, only a reduced
torque is transmitted to the connected driving wheels during the test
disconnection. This makes it possible to prevent the continuously driven
driving wheel(s) from slipping excessively during the test disconnection
and the traction power of the vehicle dropping to a comparative extent.
A differential lock operating with force locking can be controlled in the
same manner. If rotational-speed differences, (which exceed a previously
reduced threshold value) occur at the driving wheels associated with the
respective differential, even with partial detachment of the differential
lock (i.e. with reduced force locking) the differential lock is again
controlled to become fully effective. In this case there is no complete
test disconnection.
Even if the synchronism of driving wheels is achieved by separate actuation
of the wheel brakes, it is possible to reactuate an actuated wheel brake
more strongly after only partial reduction of the braking force and when a
previously reduced threshold value of a rotational-speed difference of the
driving wheels is already reached or exceeded during the partial
detachment of the said wheel brake. This ensures that the driving wheel
which is rotating more slowly in each case is continuously supplied with a
certain driving torque.
In a particularly preferred embodiment of the invention, the length of the
time interval (in which the control device responds to reduced threshold
values) depends on the frequency or the succession in time of the
threshold-value transgressions. As a result, the brakes or locks or clutch
devices respond in a particularly sensitive manner without significant
delay in slippery driving conditions.
It is also possible that the control device continuously checks which
rotational accelerations or decelerations of the vehicle wheels occur and
continuously updates corresponding measurement values and stores these for
a predetermined period of time. As long as no relatively large
rotational-speed differences between the vehicle wheels have occurred in
the rotational accelerations or decelerations registered, the registered
rotational acceleration or deceleration represents a measure of the
minimum coefficient of friction existing on the respective roadway. If a
relatively large rotational-speed difference between the different wheels
has occurred on acceleration of the vehicle, or if the anti-blocking
system has responded during braking of the vehicle, the rotational
accelerations or decelerations of the wheels, represent a measure of the
top limit of the coefficient of friction of the respective roadway. If
then, the control device changes the threshold values of the
rotational-speed differences (occurring between the vehicle wheels at
which the control device intervenes in the distribution of the drive power
to the permanent or connectable drive wheels) analogously to the magnitude
of the rotational accelerations or decelerations of the wheels occurring,
the drive system can be matched to the coefficients of friction of the
respective roadway. This is true since the control system already
intervenes in the distribution of the drive power to the permanent or
connectable driving wheels with relatively low rotational-speed
differences between the vehicle wheels on a slippery base.
In a further advantageous development of the invention, it is preferable to
change the threshold values in dependence on speed and in dependence on
the steering angle, the threshold values being reduced with increasing
value of the product of speed and steering angle. This results in the
control device intervening early in the distribution of the drive power to
the permanently driven or connectable driving wheels in tight curves, for
example when driving through mountain passes. This prevents sudden or
abrupt increased driving torques becoming effective at the driving wheels
which rotate more slowly during the intervention of the control device in
the distribution of the driving power.
This has the following significance: when driving in curves, a driving
wheel can spin particularly during acceleration. This is equivalent to the
spinning driving wheel losing its ground adherence and thus also its
cornering force while the slowly rotating driving wheel still works with
ground adherence and correspondingly existing cornering force. Preventing
a driving torque from being suddenly (abruptly) applied to the slowly
rotating driving wheel, effectively prevents the slowly rotating driving
wheel from also losing its ground adherence or cornering force due to the
intervention of the control device in the distribution of the driving
power.
In a particularly preferred embodiment of the invention, rotational-speed
differences between front and rear wheels as well as rotational-speed
differences between the wheels of opposite vehicle sides are registered.
It is thus possible, when driving quickly in curves, to control the
distribution of the drive power to the permanently driven or connectable
driving wheels primarily in dependence on rotational-speed differences
between the wheels of opposite vehicle sides. This is done by increasing
threshold values for the permissible rotational-speed differences with
increasing driving speed and increasing steering angles in order to take
into account different track radii and rotating speeds of the wheels when
driving in curves.
To avoid a change in the driving characteristics which may surprise the
driver in certain circumstances, for example the transition from
understeering to oversteering behavior, it may be suitable if the control
device opens any rear-axle differential lock which may have been actuated
(and/or the lock of the central differential between front and rear axle)
only when the steering has reached its straight-ahead position or an
adjoining position.
To facilitate the starting on a slippery base, the control device can leave
the beginning position of the locks, brakes and/or clutch arrangements,
which have been switched to be effective during a starting attempt, within
a predeterminable time interval during the previous starting attempt. This
provides that during the subsequent starting attempt, the system does not
wait until rotational-speed differences occur between the wheels of the
vehicle and, instead, a synchronism of the driving wheels is immediately
obtained so that starting can occur with the least-possible slippage.
Other objects, advantages and novel features of the present invention will
become apparent from the following detailed description of the invention
when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
The single FIGURE shows a diagrammatic top view of a vehicle with an
interconnected all-wheel drive.
DETAILED DESCRIPTION OF THE DRAWING
As a rule, the vehicle normally runs with a rear-wheel drive, that is to
say with driven rear wheels 1. The drive to the front wheels 2 is only
connected under special conditions.
The engine 3 is drive-connected via a conventional manual or automatic
transmission 4 to a divider transmission 5 which is drive-connected via a
main drive shafting 6 to the rear wheels 1 and connectable via an
auxiliary drive shafting 7 to the front wheels 2. In this arrangement, the
drive power in the main drive shafting 6 is transmitted to the rear wheels
via a rear-axle differential 8 which is provided with a lock.
In the auxiliary drive shafting 7, a front-axle differential 9 is arranged
between the steerable front wheels 2 which normally cannot be locked to
ensure good steerability of the vehicle.
The divider transmission 5 has a clutch arrangement by means of which the
auxiliary drive shafting 7 can be completely decoupled from the engine 3.
In this decoupled operating condition, the main drive shafting 6 is
directly drive-connected to the engine 3 or the transmission 4.
As soon as the auxiliary drive shafting 7 is connected, main drive shafting
6 and auxiliary drive shafting 7 are drive-connected to the two outputs of
a central differential arranged as part of the divider transmission 5, the
input of which is connected to the transmission 4 or to the engine 3,
respectively. The central differential is associated with a differential
lock so that synchronism between main drive shafting 6 and auxiliary drive
shafting 7 is forced in the locked condition.
The clutch arrangement associated with the divider transmission 5 and the
lock of the central differential of the divider transmission 5 and the
lock of the rear-axle differential 8 are automatically actuated by a
control device 10. The input of the control device 10 is connected to
various sensors. In this arrangement, sensors 11 register the rotational
speed of the front wheels 2. A sensor 12 registers the rotational speed of
the main drive shafting 6. If necessary, sensors 13 which register the
rotational speeds of the associated rear wheels 1 can also be utilized
instead of the sensor 12. Furthermore, sensors 14 and 15 are provided for
the steering angle of the front wheels 2 and the speed of the vehicle,
respectively. Sensors 16 and 17 can be provided for providing braking
signals from the respective front wheel(s) 2 and rear wheel(s) 1 to the
control device 10 when the brake is actuated.
Sensors 11 can be used for checking the extent to which the front wheels 2
rotate with different speed. Sensors 11 and 12 can be used for checking
the extent to which the mean rotational speeds of the front wheels 2 or
the rotational speed of a single front wheel 2 deviate from the mean
rotational speed of the rear wheels 1. A corresponding check can also be
performed by means of the sensors 11 and 13. Sensors 13 additionally
provide the possibility of detecting rotational-speed differences between
the rear wheels 1.
In dependence on rotational-speed differences occurring between wheels, in
dependence on the speed of the vehicle determined by means of the sensor
15, and in dependence on the steering angle sampled by means of the sensor
14, the control device 10 can select between four possibilities of
distributing the drive power of the engine 3 to the wheels 1 and 2:
In a normal case (stage 0), only the rear wheels 1 are driven and the lock
of the rear-axle differential 8 remains open.
Furthermore, a balanced all-wheel drive (stage 1) is possible in which all
wheels 1 and 2 are driven and both the lock of the central differential 5
and the lock of the rear-axle differential 8 remain open.
In addition, an all-wheel drive with a switched-on lock of the central
differential 5 is possible (stage 2), the lock of the rear-axle
differential 8 remaining open.
Finally, the vehicle can be driven with all-wheel drive and locked central
and rear-axle differentials (stage 3).
The various stages are switched on and off in dependence on different
criteria, in which arrangement the higher stages, particularly stages 2
and 3 cannot be switched on at higher vehicle driving speeds. For the
rest, the system basically switches back to stage 0 when the vehicle brake
is operated. For this purpose, the control device 10 receives a braking
signal when the brake is operated.
At very low vehicle driving speeds the vehicle continuously runs in stage
1, as a rule.
When travelling, the control device 10 continuously checks the rotational
speeds of the front and rear wheels 1 and 2, the system changing to the
respectively higher stage when threshold values are reached for
rotational-speed differences between the wheels 1 and/or 2. This stage
remains switched on for a certain holding time after which a test
disconnection occurs. If then rotational-speed differences occur again,
the higher stage is again switched on. If necessary, the system can also
change to the stage above.
To ensure an operating mode of the vehicle which is particularly matched to
the respective circumstances, the control progressively extends the
holding times if the criterion for a higher stage is again reached during
a test disconnection, or within a predeterminable time interval after a
test disconnection. In addition, or alternatively, it is provided to lower
the threshold values for a next higher stage after, or during, a test
disconnection for a predetermined period of time so that the control
device 10 responds more sensitively and thus more quickly to
rotational-speed differences which may have occurred during or after, a
test disconnection.
Special conditions can be taken into consideration by the control to
provide the desired stage.
During travelling operation, rotational-speed differences (which are not
based on differential slip of the wheels) can occur between the wheels.
Such rotational-speed differences are mainly based on different air
pressures in the tires, that is on different rolling circumferences of the
wheels. In order to provide for sensitively controlling the threshold
values for the actuation of the locks or brakes, it is desirable to be
able to take into consideration such rotational-speed differences. In this
connection, the differential speed effect can be utilized to provide a
constant quotient between the rotational speeds of these wheels because of
the different rolling circumferences of two wheels during slipless
straight-ahead driving, independently of the speed and/or acceleration of
the vehicle. Since the rotational wheel speeds are in any case
continuously monitored by sensors for an anti-blocking system of the
service brake, these signals are available without additional expenditure.
As soon as the steering assumes its straight-ahead position indicated by
the steering angle sensor, the system checks (during driving conditions
without actuation of the brakes or locks) to what extent rotational-speed
conditions of the wheels remain unchanged during speed changes of the
vehicle. This checking is effected mainly during low speeds and with
relatively low accelerations of the vehicle.
A subsequent intervention of the control device in the distribution of the
drive power to the driving wheels can occur very sensitively when there
are deviations from the previously determined rotational-speed quotient
ratio. Thus, the control device is "calibrated" for detecting a particular
rotational-speed ratio as a normal condition.
Although the present invention has been described and illustrated in
detail, it is to be clearly understood that the same is by way of
illustration and example only, and is not to be taken by way of
limitation. The spirit and scope of the present invention are to be
limited only by the terms of the appended claims.
Top